there isn't a switch for the locker, its built into the terrain response and will engage/disengage automatically varying on the setting you have the terrain response on and what inputs the ECU gets from the height sensors, wheel speed sensors, transmission, ect...
if you are looking for a truck, then you have to physically verify that the motor is on the side of the diff. It is supposed to be included with the HD package which also comes with a full size spare, but I have seen full size spares on trucks that dont have the locker and i have seen mini spares on ones that did. I have also seen supposed HD package trucks that did not have it and non HD trucks that did. heck i have seen lockers installed on trucks that didnt have the plugs to even operate it. LR is wacky sometimes.
You also CANT add the locker. the harness is integral to the body harness. and if it doesn't have it, then it doesn't have the wiring or the extra ECU to control it and whatever else it needs.
I don't remember what the screen picture dies when it locks or unlocks, but it does indicate it somehow IIRC.